Audio reproducing apparatus and audio reproduction control method for use in the same

ABSTRACT

An audio reproducing apparatus reproduces audio data to be reproduced in sync with video data. The audio reproducing apparatus includes a communication device configured to execute communication with another apparatus that reproduces the video data, a unit configured to detect an amount of an error between a reproduction point of the video data in the other apparatus and a reproduction point of the audio data in the audio reproducing apparatus by making use of the communication executed by the communication device with the other apparatus, a unit configured to detect, from a stream of the audio data, an audio data time period of audio data with a sound volume level lower than a specified value, and a unit configured to alter a reproduction speed of the audio data belonging to the detected audio data time period, on the basis of the detected amount of the error in reproduction point.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-158898, filed May 31,2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an audio reproducing apparatus forreproducing audio data that is to be reproduced in synchronism withvideo data, and to an audio reproduction control method for use in thisapparatus.

2. Description of the Related Art

In these years, techniques for altering the reproduction speed ofmultimedia data such as video and audio have been developed.

Jpn. Pat. Appln. KOKAI Publication No. 9-261613 discloses a reproducingmethod that can alter a reproduction speed in an apparatus that receivesand reproduces multimedia data. In this method, the reproduction speedis altered in accordance with the amount of residual data in a receivingbuffer. Specifically, in this method, when the amount of residual datain the receiving buffer is large, the reproduction speed is increased.When the amount of residual data in the receiving buffer is small, thereproduction speed is decreased. Thereby, overflow or underflow of thereceiving buffer is prevented.

Jpn. Pat. Appln. KOKAI Publication No. 5-135228 discloses a techniquethat realizes decelerated reproduction or accelerated reproduction ofvoice signals while maintaining high-quality reproduction, by increasingor decreasing voiceless time periods in the voice signals.

Recently, there is a demand for the advent of techniques for reproducingvideo data and audio data, which are to be reproduced in synchronism,using different apparatuses.

However, when video data and audio data are to be reproduced by separateapparatuses, the reproduction timing of the video data is controlledaccording to the time indicated by the clock provided in the apparatusfor reproducing the video data. The reproduction timing of the audiodata is controlled according to the time indicated by the clock providedin another apparatus for reproducing the audio data. The differencebetween the clock referred to for the video data reproduction timing andthe clock referred to for the audio data reproduction timing causesasynchronism between reproduced video and audio. If the reproductionspeed of video data or audio data is altered in order to eliminate theasynchronism, an unnatural impression may be given to theviewer/listener.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention provide an audio reproducingapparatus and an audio reproduction control method capable of alteringan audio data reproduction speed in accordance with a degree ofasynchronism between the audio data and video data that is reproduced byanother apparatus, without giving an unnatural impression to theviewer/listener.

According to an embodiment of the present invention, there is providedan audio reproducing apparatus that reproduces audio data to bereproduced in sync with video data, comprising: a communication deviceconfigured to execute communication with another apparatus thatreproduces the video data; a unit configured to detect an amount of aerror between a reproduction point of the video data in the anotherapparatus and a reproduction point of the audio data in the audioreproducing apparatus by making use of the communication executed by thecommunication device with the another apparatus; a unit configured todetect, from a stream of the audio data, an audio data time period ofaudio data with a sound volume level lower than a specified value; and aunit configured to alter a reproduction speed of the audio databelonging to the detected audio data time period, on the basis of thedetected amount of the error in reproduction point.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Thefeatures and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 shows an audio reproducing apparatus according to an embodimentof the present invention;

FIG. 2 shows a wireless headphone as an example that realizes the audioreproducing apparatus shown in FIG. 1;

FIG. 3 is a block diagram showing an example of the structure of a videoreproducing apparatus for reproducing video data that is to bereproduced in sync with audio data reproduced by the audio reproducingapparatus shown in FIG. 1;

FIG. 4 is a block diagram showing an example of the structure of theaudio reproducing apparatus shown in FIG. 1;

FIG. 5 is a block diagram showing the structure of an audio reproductionspeed control unit provided in the audio reproducing apparatus shown inFIG. 1;

FIG. 6 is a view for explaining a sound volume level detection processexecuted by an audio output level detection unit provided in the audioreproducing apparatus shown in FIG. 1;

FIG. 7 is a flow chart illustrating an audio reproduction controlprocess executed by the audio reproducing apparatus shown in FIG. 1;

FIG. 8 shows an example of rules to be used when the audio reproducingapparatus shown in FIG. 1 determines an amount of change in audioreproduction speed;

FIG. 9 shows a state in which the audio data reproduction speed isaltered in the audio reproducing apparatus shown in FIG. 1; and

FIG. 10 is a block diagram showing an example of a system wherein audiodata is sent from a server to the audio reproducing apparatus.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described withreference to the accompanying drawings.

FIG. 1 shows an audio reproducing apparatus 20 according to anembodiment of the invention. The audio reproducing apparatus 20 isconfigured to reproduce audio data in synchronism with the reproductionof video data, such as moving picture, by a video reproducing apparatus10. For example, when audio/video content, which includes video data andaudio data to be reproduced in sync with the video data, is to bereproduced, the video reproducing apparatus 10 performs video datareproduction while the audio reproducing apparatus 20 performs audiodata reproduction.

The audio reproducing apparatus 20, while receiving to-be-reproducedaudio data from an external apparatus over a communication path by wireor by radio, reproduces the received audio data. Audio data is sent tothe audio reproducing apparatus 20 by one of the following methods: (1)to send audio data from the video reproducing apparatus 10 to the audioreproducing apparatus 20, or (2) to send video data and audio datasimultaneously from an external content server to the video reproducingapparatus 10 and audio reproducing apparatus 20, respectively. In thedescription below, method (1) is adopted.

When audio-video content is to be reproduced, the video reproducingapparatus 10 separates the audio-video content into video data and audiodata. The video reproducing apparatus 10 reproduces the video data whiletransmitting the audio data to the audio reproducing apparatus 20. Ashort-distance wireless communication protocol represented by Bluetooth™is used for audio data transmission from the video reproducing apparatus10 to the audio reproducing apparatus 20. In this case, the videoreproducing apparatus 10 transmits the audio data wirelessly from anantenna 101 to the audio reproducing apparatus 20, and the audioreproducing apparatus 20 receives the audio data via an antenna 201 fromthe video reproducing apparatus 10. The audio reproducing apparatus 20is realized as a wireless headphone, as shown in FIG. 2.

The audio reproducing apparatus 20, while receiving the audio data fromthe video reproducing apparatus 10, reproduces the received audio data.During a time period in which the audio data is being reproduced, theaudio reproducing apparatus 20 periodically communicates with the videoreproducing apparatus 10 and detects an amount of an error between areproduction point of video data in the video reproducing apparatus 10and a reproduction point of audio data in the audio reproducingapparatus 20 (i.e. an amount of asynchronism between video and audio).In order to realize synchronous reproduction of video and audio, theaudio reproducing apparatus 20 alters the reproduction speed of audiodata in accordance with the detected amount of asynchronism. Thealteration of reproduction speed of audio data is effected in a timeperiod of an audio data component in which a sound volume level of audiois lower than a predetermined value.

Referring to FIGS. 3 and 4, the structures of the video reproducingapparatus 10 and audio reproducing apparatus 20 will now be described.

FIG. 3 shows a system configuration of the video reproducing apparatus10. The video reproducing apparatus 10 is realized, for example, as aportable electronic apparatus such as a PDA (Personal DigitalAssistant), a portable personal computer or a mobile phone.

As is shown in FIG. 3, the video reproducing apparatus 10 comprises aCPU 11, a time management unit 12, a memory device 13, a wirelesscommunication device 14, a video/audio separation unit 15, a decodingunit 16, a video reproducing unit 17, and a display 18.

The CPU 11 is a processor that is provided to control operations of thevideo reproducing apparatus 10. The CPU 11 is connected to othercomponents of the video reproducing apparatus 10 over a bus 100. The CPU11 cooperates with these components. The time management unit 12comprises a clock device that measures time. The time management unit 12provides time information which serves as a reference time for the videodata reproducing operation.

The memory device 13 comprises, for instance, a non-volatile memorydevice, and stores audio-video content to be reproduced. The audio-videocontent is composed of a stream of encoded video data and a stream ofencoded audio data to be reproduced in sync with the video data stream.The video data stream and audio data stream are multiplexed. AVI (AudioVideo Interleave Format), for instance, may be used as a data format ofaudio-video content. In AVI format content data, sampling rates arerecorded for associated video and audio media, respectively.

The video/audio separation unit 15 separates the to-be-reproducedcontent data into video data and audio data. The video data separatedfrom the content data by the video/audio separation unit 15 is sent tothe decoding unit 16, while the audio data separated from the contentdata by the video/audio separation unit 15 is sent to the wirelesscommunication device 14.

The wireless communication device 14 establishes wireless connectionwith the audio reproducing apparatus 20 and performs wirelesscommunication with the audio reproducing apparatus 20. The wirelesscommunication is executed according to procedures based on, forinstance, Bluetooth™ protocol. The wireless communication device 14,under control of CPU 11, transmits the encoded audio data to the audioreproducing apparatus 20 by radio signals.

The decoding unit 16 decodes the encoded video data and decompresses thecompressed video data into a displayable format. The video reproducingunit 17 controls the display 18 and executes a reproducing operation forenabling the display 18 to display the video data decoded by thedecoding unit 16. In this case, the reproduction speed of video data iscontrolled by the time information managed by the time management unit12 and the sampling rate of the video data.

The reproduction of the video data by the video reproducing unit 17begins after a predetermined time period has passed since the start of aprocess for sending the audio data to the audio reproducing apparatus 20by radio signals. This predetermined time period is set in advance onthe basis of the time needed until the audio reproducing apparatus 20 ismade ready to reproduce the audio data. It is thus possible tosimultaneously start the reproduction of audio data by the audioreproducing apparatus 20 and the reproduction of video data by the videoreproducing unit 17.

FIG. 4 shows a system configuration of the audio reproducing apparatus20. The audio reproducing apparatus 20 is realized as a wirelessheadphone, as shown in FIG. 2. The audio reproducing apparatus 20, asshown in FIG. 4, comprises a CPU 21, a time management unit 22, awireless communication device 23, a decoding unit 24, a buffer memory25, an audio reproducing unit 26, an audio output unit 27, a videoreproduction point acquiring unit 28, an asynchronism amount detectionunit 29, a sound output level detection unit 30, and an audioreproduction speed control unit 31.

The CPU 21 is a processor that is provided to control operations of theaudio reproducing apparatus 20. The CPU 21 is connected to othercomponents of the audio reproducing apparatus 20 over a bus 200. The CPU21 cooperates with these components. The time management unit 22comprises a clock device that measures time. The time management unit 22provides time information which serves as a reference time for the audiodata reproducing operation.

The wireless communication device 23 establishes wireless connectionwith the video reproducing apparatus 10 and performs wirelesscommunication with the video reproducing apparatus 10. The wirelesscommunication is executed according to procedures based on, forinstance, Bluetooth™ protocol. The wireless communication device 23,under control of CPU 21, receives the encoded audio data from the videoreproducing apparatus 10.

The decoding unit 24 decodes the encoded audio data received by thewireless communication device 23 and decompresses the compressed audiodata into a reproducible format. The decoded audio data is stored in thebuffer memory 25. The buffer memory 25 is a memory device fortemporarily storing audio data to be reproduced. The buffer memory 25stores audio data having a length corresponding to a reproduction timeof, e.g. about 50 msec to 600 msec.

The audio reproducing unit 26 reproduces the audio data stored in thebuffer memory 25 and outputs it as sound via the audio output unit 27comprising a loudspeaker or a headphone. The audio reproducing operationinvolves, for example, a process for converting the audio data stored inthe buffer memory 25 from a digital signal to an analog signal. Thereproduction speed of audio data is basically controlled by the timeinformation managed by the time management unit 22 and the sampling rateof the audio data, but it can be altered by the control under the audioreproduction speed control unit 31.

The clock (time information) referred to in the control of the audioreproduction timing in the audio reproduction apparatus 20 differs fromthe clock (time information) referred to in the control of the videoreproduction timing in the video reproducing apparatus 10. As a result,asynchronism may occur between audio reproduced by the audio reproducingapparatus 20 and video reproduced by the video reproducing apparatus 10.If the same clock can be referred to in the video reproduction timingcontrol and the audio reproduction timing control, it is easy to realizesynchronous reproduction of audio and video. However, in the system,like the present embodiment, wherein the apparatus for reproducing videoand the apparatus for reproducing audio are physically different, it ispractically difficult for one apparatus to frequently refer to the clockin the other apparatus.

As mentioned above, in the present embodiment, the audio reproductiontiming control is executed referring to a clock (time information oftime management unit 22) different from the clock for the videoreproduction timing control. While the audio is being reproduced, theaudio reproducing apparatus 20 periodically acquires a current videoreproduction point from the video reproducing apparatus 10 making use ofradio communication with the video reproducing apparatus 10. Based onthe acquired video reproduction point, the audio reproducing apparatus20 corrects the audio reproduction point and thus eliminatesasynchronism.

The correction process for the audio reproduction point is executed bythe video reproduction point acquiring unit 28, asynchronism amountdetection unit 29, sound output level detection unit 30, and audioreproduction speed control unit 31.

Making use of the radio communication performed by the wirelesscommunication device 23 with the video reproducing apparatus 10, thevideo reproduction point acquiring unit 28 and asynchronism amountdetection unit 29 detect the amount of an error between the reproductionpoint in video data in the video reproducing apparatus 10 and thereproduction point in audio data in the audio reproducing apparatus 20.

Specifically, using the wireless communication device 23, the videoreproduction point acquiring unit 28 sends a request for acquiring avideo reproduction point to the video reproducing apparatus 10 andacquires the current reproduction point in the video data from the videoreproducing apparatus 10. The request for acquiring the videoreproduction point is periodically sent to the video reproducingapparatus 10 while the audio data is being reproduced. The asynchronismamount detection unit 29 detects the amount of an error of a relativeposition of audio data to video data (i.e. the amount of asynchronism)on the basis of the current reproduction point in the audio datareproduced by the audio reproducing unit 26 and the current reproductionpoint in the video data acquired by the video reproduction pointacquiring unit 28.

The current reproduction point in audio data is given by an elapsed timefrom the start of reproduction of audio data up to now. Similarly, thecurrent reproduction point in video data is given by an elapsed timefrom the start of reproduction of video data up to now.

The asynchronism amount detection process by the video reproductionpoint acquiring unit 28 and asynchronism amount detection unit 29 can beexecuted by utilizing clock synchronization according to, e.g. NTP(Network Time Protocol). The NTP is a protocol for making systems, whichare connected via networks, have the same time. Specifically, the videoreproducing apparatus 10 and audio reproducing apparatus 20 control thevideo and audio reproduction timings, referring to their own timemanagement units 12 and 22. The difference between the time indicated bythe time management unit 22 of audio reproducing apparatus 20 and thetime indicated by the time management unit 12 of video reproducingapparatus 10 is checked using the NTP. Thereby, the amount of errorbetween the current reproduction point of video data and the currentreproduction point of audio data can be detected.

The sound output level detection unit 30 analyzes the audio data streamstored in the buffer memory 25, thereby detecting the sound volume level(also referred to as “sound output level”) of the audio data stream thatis about to be reproduced. This sound volume level detection processdetects, from the audio data stream to be reproduced, a time period ofan audio data component with a sound volume level lower than apredetermined value.

The audio reproduction speed control unit 31 is configured to execute acontrol to alter the audio data reproduction speed of the audioreproducing unit 26 in order to eliminate asynchronism. The alterationof audio reproduction speed is effected for the audio data component ofthe to-be-reproduced audio data, which belongs to the detected timeperiod in which the sound volume level is low. The reason is that if amajor alteration of the audio reproduction speed is effected in the timeperiod of an audio data component with a high sound volume level, anunnatural impression may possibly given to the viewer/listener. In thetime period of an audio data component with a low sound volume level,alteration of the audio reproduction speed may less possibly cause anunnatural impression to the user.

The audio reproduction speed control unit 31, as shown in FIG. 5,comprises an audio reproduction speed determination unit 311 and anaudio reproduction speed alteration unit 312. The audio reproductionspeed determination unit 311 determines the amount of alteration of theaudio reproduction speed on the basis of the amount of asynchronismdetected by the asynchronism amount detection unit 29 and the detectedresult of the sound volume level by the sound output level detectionunit 30. The amount of alteration of the audio reproduction speed isdetermined according to preset rules so that the audio reproductionspeed may be altered in the time period in which the sound volume levelis relatively low. These rules specify a maximum tolerable alterationamount of the audio reproduction speed for each of sound volume levels.

The audio reproduction speed alteration unit 312 alters the audio datareproduction speed in the audio reproducing unit 26 in accordance withthe alteration amount of audio reproduction speed determined by theaudio reproduction speed determination unit 311. The alteration of theaudio reproduction speed is effected in three processes: (1) toincrease/decrease the amount of audio data to be reproduced per unittime, (2) to decimate (or delete) audio data, and (3) to insert specificaudio data (silent data). Process (1) is a process of increasing ordecreasing the audio data reproduction speed, without decimating orinserting data.

In order to execute processes (1), (2) and (3) for the audioreproduction speed alteration, the audio reproduction speed alterationunit 312 includes a speed adjustment unit 313, a data decimation unit314 and a silent data insertion unit 315. The speed adjustment unit 313performs a process of increasing or decreasing the amount of audio datato be reproduced per unit time relative to a reference amount, therebyaltering the audio data reproduction speed without decimating orinserting data. The reference amount is determined based on the samplingrate of audio data and the clock output from the time management unit22. The data decimation unit 314 and silent data insertion unit 315perform processes of decimating the audio data and inserting silent datain the audio data, respectively.

Assume that the reproduction point of video data precedes thereproduction point of audio data. In this case, in order to increase theaudio data reproduction speed, the audio reproduction speed control unit31 increases the amount of audio data to be reproduced per unit time ordecimates (e.g. deletes) audio data, thereby eliminating theasynchronism. On the other hand, assume that the reproduction point ofvideo data is behind the reproduction point of audio data. In this case,in order to decrease the audio data reproduction speed, the audioreproduction speed control unit 31 decreases the amount of audio data tobe reproduced per unit time or inserts specific data (e.g. silent data),thereby eliminating the asynchronism.

An example of the sound volume level detection process will now bedescribed.

FIG. 6 schematically shows audio data stored in the buffer memory 25.The sound volume level of audio data is given by a magnitude of waveformof a digital audio signal, which is represented by a line connectingtime-sequential sample data elements (indicated by black dots in FIG.6). Each sample data element is obtained by sampling the analog audiosignal at a sampling rate of, e.g. 44.1 KHz.

The sound output level detection unit 30 examines, e.g. a variation inamplitude of the audio data waveform, and thus detects an audio timeperiod with “volume level=high”, an audio time period with “volumelevel=medium” and an audio time period with “volume level=low” in thestream of the audio data stored in the buffer memory 25. The audioreproduction speed of the audio data belonging to the audio time periodwith “volume level=medium” or “volume level low” is altered. The audioreproduction speed of the audio data belonging to the audio time periodwith “volume level=high” is not altered. The maximum tolerablealteration amount of the audio reproduction speed in the audio timeperiod with “volume level=medium” is smaller than that of the audioreproduction speed in the audio time period with “volume level=small.”

The detection of the sound output level uses a first threshold and asecond threshold that is lower than the first threshold, with which thethree time periods with “volume level=high”, “volume level=medium” and“volume level=low” are detected. A time period of audio data, whoseamplitude, i.e. sound volume level, is lower than the second threshold,is detected as the time period with “volume level=low.” A time period ofaudio data, whose amplitude, i.e. sound volume level, is between thefirst and second thresholds, is detected as the time period with “volumelevel=middle.” A time period of audio data, whose amplitude, i.e. soundvolume level, is higher than the first threshold, is detected as thetime period with “volume level=high.”

The first and second thresholds are predetermined fixed values.

A description will now be given of the operation of the audioreproduction control process executed by the audio reproductionapparatus 20.

A flow chart of FIG. 7 illustrates procedures by which the audioreproducing apparatus 20 detects asynchronism between video and audioand alters the audio reproduction speed on the basis of the detectionresult.

The audio reproducing apparatus 20, while receiving audio data from thevideo reproducing apparatus 10, reproduces the audio data. During theaudio data reproduction time period, the audio reproducing apparatus 20executes a synchronization process at predetermined time intervals,thereby to reproduce the audio in sync with the video reproduction inthe video reproducing apparatus 10.

Specifically, the audio reproducing apparatus 20 determines whethersynchronism needs to be established, on the basis of an elapsed timefrom the last synchronization process (step S101). If the elapsed timefrom the last synchronization process is a predetermined time or more(YES in step S101), the audio reproducing apparatus 20 performs thefollowing synchronization process.

To begin with, making use of the communication between the wirelesscommunication device 23 and video reproducing apparatus 10, the videoreproduction point acquiring unit 28 of audio reproducing apparatus 20acquires the reproduction point of video currently reproduced by thevideo reproducing apparatus 10 (step S102). Then, the asynchronismamount detection unit 29 compares the video reproduction point in thevideo reproducing apparatus 10 acquired in step S102 and the audioreproduction point in the audio reproducing apparatus 20, and determineswhether there is an error between the reproduction point of video andreproduction point of audio (step S103).

If there is an error between the reproduction point of video andreproduction point of audio (YES in step S103), the sound output leveldetection unit 30 of audio reproducing apparatus 20 performs soundvolume level detection for the audio data stream stored in the buffer25, which follows the reproduction point of audio that is currentlyreproduced (step S104). Subsequently, the audio reproduction speeddetermination unit 311 of audio reproduction speed control unit 31determines the time period of audio data, whose reproduction speed is tobe altered, and the amount of alteration in reproduction speed in thistime period, according to preset rules, on the basis of the error inreproduction point between video and audio, which has been detected instep S103, and the sound volume level of audio data detected in stepS104 (step S105).

FIG. 8 shows an example of the rules used in determining the amount ofalteration in audio reproduction speed. In the table of FIG. 8, thefirst row shows the sound volume levels of audio data detected in stepS104. The second row indicates maximum tolerable alteration amounts ofaudio reproduction speed, which are associated with the sound volumelevels in the first row, that is, maximum tolerable increase/decreaseranges of the data amount of audio to be reproduced per unit time.Similarly, the third row indicates maximum tolerable times for audiodata insertion or decimation (deletion), which are associated with thesound volume levels in the first row, that is, time periodscorresponding to maximum tolerable data lengths of audio data which canbe inserted or deleted per unit time.

The maximum tolerable alteration amount of audio reproduction speed andthe maximum tolerable time for audio data insertion/deletion take highervalues as the sound volume level becomes lower, and take lower values asit becomes higher.

The alteration of audio reproduction speed or the insertion/deletion ofaudio data is repeated more than once until the asynchronism betweenvideo and audio is eliminated. If the asynchronism is not eliminatedwithin a predetermined time period from the start of the audioreproduction speed alteration, the audio data is immediately deleted orinserted by the amount corresponding to the time necessary foreliminating the asynchronism, irrespective of the sound volume level ofaudio data to be reproduced.

In order to eliminate asynchronism between video and audio in the audioreproducing apparatus 20, the audio reproduction point is corrected withreference to the video reproduction point. If the reproduction point ofvideo data precedes the reproduction point of audio data, the amount ofaudio data to be reproduced per unit time is increased or the audio datais deleted, thereby eliminating the asynchronism. On the other hand, ifthe reproduction point of video data is behind the reproduction point ofaudio data, the amount of audio data to be reproduced per unit time isdecreased or audio data (silent data) is inserted, thereby eliminatingthe asynchronism.

In step S105, in this manner, the amount of increase/decrease of audiodata to be reproduced per unit time, or the time for audio datainsertion/deletion, is determined. In accordance with the determination,the audio reproduction speed alteration unit 312 of audio reproductionspeed control unit 31 controls the audio reproducing unit 26, and altersthe audio reproduction speed of audio reproducing unit 26 (step S106).

An example of the process in steps S104 to S106 will now be describedwith reference to FIG. 9.

FIG. 9 shows a state in which the audio data reproduction speed isaltered. FIG. 9 shows three audio data waveforms. The upper audio datawaveform indicates a normal state with no asynchronism. The middle audiodata waveform indicates a state in which asynchronism has occurred andthe audio data reproduction is behind the video data reproduction. Thelower audio data waveform indicates a state in which asynchronism hasbeen eliminated by the process of altering the audio reproduction speed.

Assume that a delay of 100 msec has detected at time instant A in FIG.9. In this case, the sound volume level of audio data in the buffermemory 25, which follows the time instant A, is detected (step S104).Based on the detected sound volume level, the amount of alteration ofthe audio reproduction speed is determined according to the rules shownin FIG. 8 (step S105).

The sound volume level of the audio data belonging to a time period (100msec) between time instant A and time instant B is “high”. Thus,according to the rules, the amount of alteration of the audioreproduction speed in the time period between time instant A and timeinstant B is 0 msec. However, the sound volume level of the audio databelonging to a time period (600 msec) between time instant B and timeinstant D is “low”. Thus, according to the rules, insertion ordecimation of audio data in a range of ±500 msec per second ispermitted. The detected delay is 100 msec. This delay can be eliminatedby deleting audio data corresponding to 100 msec from the audio databelonging to the time period (600 msec) between time instant B and timeinstant D. Specifically, the audio data in the time period (600 msec)between time instant B and time instant D is reduced to 500 msec.Thereby, the reproduction point of audio data is made to coincide withthe reproduction point of video data at time instant C.

When asynchronism is to be detected by the NTP, it is preferable toadvance the time of the time management unit 22 by 100 msec at the timeinstant C at which the error in reproduction point is eliminated.

As has been described above in detail, according to the presentembodiment, the amount of asynchronism can be suppressed within apredetermined range by the audio reproduction speed alteration process,even without strictly making the clock in the video reproducingapparatus 10 coincide with the clock in the audio reproducing apparatus20. Therefore, even in a system wherein video and audio are reproducedby different apparatuses, a so-called lip-sync can easily be achieved.In particular, since the audio reproduction speed is altered in the timeperiod in which the sound volume level of audio data is low, lip-synccan be realized without giving unnatural impression to theviewer/listener.

It is not necessary that the video reproducing apparatus 10 sends audiodata to the audio reproducing apparatus 20. For example, as shown inFIG. 10, a content server 40 may send video data and audio data to thevideo reproducing apparatus 10 and audio reproducing apparatus 20 byradio signals via an antenna 301. In this case, too, the audioreproducing apparatus 20 may periodically acquire a video reproductionpoint from the video reproducing apparatus 10, thereby detectingasynchronism between video and audio.

The audio reproduction control process in the audio reproducingapparatus 20 according to the present embodiment may be realized bycomputer-executable programs. In this case, a computer-readable storagemedium storing computer programs including procedures of theabove-described audio reproduction control process is prepared. Thecomputer programs are installed in an ordinary computer with acommunication function and executed, and thereby the same advantages aswith the present embodiment can easily be obtained.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An audio reproducing apparatus that reproduces audio data to bereproduced in sync with video data, comprising: a communication deviceconfigured to execute communication with another apparatus thatreproduces the video data; a unit configured to detect an amount of anerror between a reproduction point of the video data in said anotherapparatus and a reproduction point of the audio data in the audioreproducing apparatus by making use of the communication executed by thecommunication device with said another apparatus; a unit configured todetect, from a stream of the audio data, an audio data time period ofaudio data with a sound volume level lower than a specified value; and aunit configured to alter a reproduction speed of the audio databelonging to the detected audio data time period, on the basis of thedetected amount of the error in reproduction point.
 2. The audioreproducing apparatus according to claim 1, wherein the unit that altersthe reproduction speed includes a unit that decimates audio databelonging to the detected audio data time period.
 3. The audioreproducing apparatus according to claim 1, wherein the unit that altersthe reproduction speed includes a unit that inserts silent data in thedetected audio data time period.
 4. The audio reproducing apparatusaccording to claim 1, wherein the unit that alters the reproductionspeed includes a unit that alters an amount of audio data to bereproduced per unit time, the audio data belonging to the detected audiodata time period.
 5. The audio reproducing apparatus according to claim1, wherein the unit that detects an audio data time period of audio datawith a sound volume level lower than a specified value is configured todetect, from the stream of the audio data, a first audio data timeperiod of audio data with a sound volume level falling within a rangebetween a first sound volume level and a second sound volume level lowerthan the first sound volume level, and a second audio data time periodof audio data with a sound volume level lower than the second soundvolume level, and the unit that alters the reproduction speed includes aunit that determines an alteration amount of reproduction speed of audiodata belonging to each of the first and second audio data time periods,on the basis of a maximum tolerable alteration amount relating to thereproduction speed of audio data, which is specified for each of thefirst and second audio data time periods, and the detected amount of theerror in reproduction point.
 6. The audio reproducing apparatusaccording to claim 1, wherein the communication device is configured toexecute communication with said another apparatus using radio signals.7. The audio reproducing apparatus according to claim 1, wherein thecommunication device is configured to execute communication with saidanother apparatus, thereby receiving the audio data from said anotherapparatus, the audio reproducing apparatus further comprises a memorydevice that temporarily stores the received audio data, and the unitthat detects an audio data time period of audio data with a sound volumelevel lower than a specified value is configured to detect, from thestream of the audio data stored in the memory device, an audio data timeperiod of audio data with a sound volume level lower than the specifiedvalue.
 8. The audio reproducing apparatus according to claim 7, whereinthe communication device is configured to execute communication withsaid another apparatus using radio signals.
 9. The audio reproducingapparatus according to claim 1, wherein the unit that detects the amountof an error includes: means for acquiring a current reproduction pointof video data from said another apparatus by using the communicationdevice; and means for comparing the acquired reproduction point of videodata and the reproduction point of the audio data in the audioreproducing apparatus.
 10. An audio reproduction control method forcontrolling a reproduction operation of an audio reproducing apparatusthat reproduces audio data to be reproduced in sync with video data,comprising: executing communication with another apparatus thatreproduces the video data, thereby detecting an amount of an errorbetween a reproduction point of the video data in said another apparatusand a reproduction point of the audio data in the audio reproducingapparatus; detecting, from a stream of the audio data, an audio datatime period of audio data with a sound volume level lower than aspecified value; and altering a reproduction speed of the audio databelonging to the detected audio data time period, on the basis of thedetected amount of the error in reproduction point.
 11. The audioreproduction control method according to claim 10, wherein the alteringof the reproduction speed includes decimating audio data belonging tothe detected audio data time period.
 12. The audio reproduction controlmethod according to claim 10, wherein the altering of the reproductionspeed includes inserting silent data in the detected audio data timeperiod.
 13. The audio reproduction control method according to claim 10,wherein the altering of the reproduction speed includes altering anamount of audio data to be reproduced per unit time, the audio databelonging to the detected audio data time period.
 14. The audioreproduction control method according to claim 10, wherein the detectingof an audio data time period of audio data with a sound volume levellower than a specified value includes detecting, from the stream of theaudio data, a first audio data time period of audio data with a soundvolume level falling within a range between a first sound volume leveland a second sound volume level lower than the first sound volume level,and a second audio data time period of audio data with a sound volumelevel lower than the second sound volume level, and the altering of thereproduction speed includes determining an alteration amount ofreproduction speed of audio data belonging to each of the first andsecond audio data time periods, on the basis of a maximum tolerablealteration amount relating to the reproduction speed of audio data,which is specified for each of the first and second audio data timeperiods, and the detected amount of the error in reproduction point. 15.The audio reproduction control method according to claim 10, wherein thecommunication with said another apparatus is executed by wirelesscommunication.
 16. The audio reproduction control method according toclaim 10, further comprising: receiving the audio data from said anotherapparatus via a communication device provided in said another apparatus,and temporarily storing the received audio data in a memory deviceprovided in the audio reproducing apparatus, and wherein the detectingof an audio data time period of audio data with a sound volume levellower than a specified value includes detecting, from the stream of theaudio data stored in the memory device, an audio data time period ofaudio data with a sound volume level lower than the specified value. 17.The audio reproduction control method according to claim 16, wherein theaudio data is sent from said another apparatus by radio signals.
 18. Theaudio reproduction control method according to claim 10, wherein thedetecting of the amount of an error includes: acquiring a currentreproduction point of video data from said another apparatus bycommunication with said another apparatus at predetermined timeintervals; and comparing the acquired reproduction point of video dataand the reproduction point of the audio data in the audio reproducingapparatus.